1. Field of the Invention
The present invention generally relates to a reflection-type liquid crystal display device (LCD) for providing display by reflecting light incident from the outside. More particularly, the present invention relates to a reflection-type LCD having at least two liquid crystal layers for selectively reflecting light in a specific wavelength range.
2. Description of the Background Art
At present, liquid crystal display devices (LCDs) characterized by thin thickness, light weight and the like are practically used as color display devices in applications like office automation (OA) equipments (such as word processors and notebook computers), various video equipments, and game-playing equipments. Especially, a transmission-type LCD having a light source at the back (backlight) is used in a wide variety of fields because of the above characteristics.
Unlike the transmission-type LCD, a reflection-type LCD requires no backlight for display, thereby enabling reduction in power consumption for a light source. Moreover, since the space for the backlight is not required, the weight and thickness can further be reduced. The reflection-type LCD thus enables reduction in power consumption and therefore is suitable for lightweight, thin equipments. It is now assumed that an equipment having the transmission-type LCD and an equipment having the reflection-type LCD have the same operation time. The reflection-type LCD does not require the space and weight of the backlight and consumes a smaller amount of power. Therefore, a smaller battery can be used in the reflection-type LCD, thereby enabling further reduction in size and weight. Alternatively, provided that the equipment having the transmission-type LCD and the equipment having the reflection-type LCD have the same size or weight, a battery with a greater capacity can be used in the reflection-type LCD, whereby significant improvement in operation time can be expected.
In terms of display contrast characteristics, display devices such as CRT (Cathode Ray Tube) as a light-emitting display device are subjected to significant reduction in contrast when they are used outdoor in the daytime. Even in a transmission-type LCD subjected to an anti-reflection process, significant reduction in contrast is inevitable if the intensity of ambient light such as direct sunlight is much higher than that of display light. On the other hand, in the reflection-type LCD, display light is proportional to the amount of ambient light. Therefore, the reflection-type LCD can be used in a preferable manner particularly in outdoor applications such as portable information terminal, digital camera, and portable video camera.
The reflection-type LCDs thus have a very promising field of application. However, the reflection-type LCDs that are currently used in practical applications have a low reflectance (the ratio of reflected light intensity to incident light intensity). Therefore, display provided by the reflection-type LCDs is not bright enough. Such a low reflectance is mainly caused by the following factor: the reflection-type LCDs that are currently used in practical applications use one or twopolarizers whether they are of TN (Twisted Nematic) type or STN (Super Twisted Nematic) type. These polarizers absorb 50% or more of incident light. The absorbed light will not be used for display, causing light losses.
In view of this, a reflection-type LCD having a liquid crystal layer for selectively reflecting light in a visible light range (a liquid crystal layer having a helical structure such as a cholesteric liquid crystal layer) is conventionally proposed as a reflection-type LCD using no polarizer.
The phenomenon that this cholesteric layer selectively reflects light at a wavelength corresponding to its helical pitch is known in literatures (Appl. Opt. Vol. 7, 9, pp. 1729-1737 (1968) and Phys. Rev. Vol. 25, 9, pp. 577-581 (1970)) and the like. More specifically, provided that xe2x80x9cnoxe2x80x9d and xe2x80x9cnexe2x80x9d are a refractive index of a liquid crystal layer for ordinary ray and extraordinary ray, respectively, and xe2x80x9cpxe2x80x9d is a helical pitch, and xe2x80x9cxcexxe2x80x9d is a reflection wavelength, a right-handed cholesteric liquid crystal layer selectively reflects only right-handed circularly polarized light components of incident light having a wavelength xcex (noxc2x7p less than xcex less than nexc2x7p), and allows right-handed circularly polarized light components at other wavelengths and all left-handed circularly polarized light components to transmit therethrough. A median reflection wavelength xcexm is given by xcexm=naxc2x7p, where xe2x80x9cnaxe2x80x9d is an average refractive index of the liquid crystal layer. A left-handed cholesteric liquid crystal layer functions in the opposite manner to that of the right-handed cholesteric liquid crystal layer.
A typical example of a liquid crystal material having selective reflection characteristics is a cholesteric liquid crystal material. In order to use the selective reflection characteristics of the cholesteric liquid crystal material for display, the cholesteric liquid crystal material is commonly aligned in a planar state so that efficient selective reflection is realized. Accordingly, applying the planar-aligned cholesteric liquid crystal material to a reflection-type LCD would enable implementation of highly bright display in the regular reflection direction of a light source (the direction in which light from the light source reflects at the same angle as the incident angle).
However, reflected light intensity is reduced in an oblique direction with respect to the regular reflection direction of the light source. Moreover, the color of the reflected light varies toward a shorter wavelength as the incident angle or the reflection angle is increased. As a result, color purity of the reflected light is degraded as the viewing angle is increased. This phenomenon is called variation in hue. Moreover, if the reflected light intensity significantly varies depending on the viewing angle, that is, if the reflectance varies sharply, reflection characteristics like metallic luster texture are recognized if the display device is viewed from the regular reflection direction of the light source or a direction close to the regular reflection direction. This is not preferable in terms of display quality.
In view of the above problems, it is an object of the present invention to provide a reflection-type LCD having excellent visibility in a wide viewing-angle range.
According to the present invention, a reflection-type liquid crystal display device (LCD) includes at least two liquid crystal layers for selectively reflecting light in a specific wavelength range, and at least one partition wall interposed between the at least two liquid crystal layers so as to separate the liquid crystal layers from each other. Any one of the partition wall, a part of the liquid crystal layer which contacts the partition wall, and a part of the liquid crystal layer which contacts an alignment layer laminated to the partition wall serves as a scattering layer having a light scattering function. Whether the form of the xe2x80x9cscattering layerxe2x80x9d of the present invention can be distinguished or not does not matter as long as it has a light scattering function. For example, if individual domains of liquid crystal molecules in a region of a liquid crystal layer which is located near the interface between the partition wall and the liquid crystal layer are reduced in size by the influence of an alignment film or the like, scattering may occur between the domains. Provided that a part of the liquid crystal layer that contacts the partition wall has a light scattering function, this part of the liquid crystal layer serves as a xe2x80x9cscattering layerxe2x80x9d even if this part of the liquid crystal layer does not form a layer in the liquid crystal layer in a distinguishable form. Note that the xe2x80x9cdomainxe2x80x9d herein refers to a region occupied by regularly aligned liquid crystal molecules.
According to the reflection-type LCD of the present invention, any one of the partition wall, a part of the liquid crystal layer which contacts the partition wall, and a part of the liquid crystal layer which contacts an alignment layer laminated to the partition wall serves as a scattering layer for scattering light. The light scattering function of the scattering layer causes scattering of not only incident light but also reflected light. Therefore, excellent visibility can be obtained not only in the regular reflection direction but also in a wide viewing-angle range. Moreover, since the reflectance changes gradually with a change in viewing angle, metallic luster of the display texture is suppressed.
A liquid crystal layer that selectively reflects light in a specific wavelength range, e.g., a cholesteric liquid crystal layer, has such a property that the spectrum of selectively reflected light varies depending on the incident angle of light. In other words, as a polar angle of the incident light (tilt angle with respect to the normal of the substrate) is increased, the selectively reflected light is shifted toward a shorter wavelength. As shown in FIG. 12, providing a scattering layer 70 allows light 110a, 110b, and 110c to be incident on the liquid crystal layer 30b at various polar angles and to be reflected at the respective polar angles. Light 113a, 113b, 113c thus reflected is scattered again in the scattering layer 70. Of the scattered reflected light 113a to 113c, light components scattered in a certain direction are observed as combined light 113d. Therefore, the observed color is an average of various spectra. Accordingly, even if the display device is viewed in an oblique direction with respect to the normal direction of the substrate, perceived variation in hue toward a shorter wavelength is reduced as compared to the case of selective reflection without the scattering layer 70.
Moreover, the partition wall or the alignment layer provides a light scattering function. This eliminates the need to form a separate scattering layer, whereby increase in the number of parts, the number of steps in the manufacturing process and the manufacturing costs can be suppressed.
In the reflection-type LCD of the present invention, the at least two liquid crystal layers are preferably two or three liquid crystal layers. Preferably, the liquid crystal layers selectively reflect different specific wavelength ranges and have different threshold voltages. This enables implementation of four-color or eight-color display.
In the reflection-type LCD of the present invention, a scattered light intensity I (xcex8) of the scattering layer is preferably distributed such that an average value of I (0xc2x0) to I (10xc2x0) is equal to or less than 500 times an average value of I (20xc2x0) to I (30xc2x0). xe2x80x9cxcex8xe2x80x9d in the scattered light intensity I (xcex8) is a light-receiving angle with respect to the direction in which parallel light from a light source travels straight. Therefore, the scattered light intensity I (xcex8) indicates a scattered light intensity at the light-receiving angle xcex8. Since the reflectance changes gradually with a change in viewing angle, metallic luster of the display texture can be suppressed.